Crank hardening after polishing ????

My crank needs to be polished. I am told that forged cranks are sonic hardened and this hardening is very thin and any abrasive will cut threw it. So is this true? Can most shops re-apply sonic hardening or some other better method of hardening.

It's not sonic hardening, it's nitride or surface hardening. The stock crank is surface hardened I believe. There is a reasonable amount of material that can be removed without endangering the surface hardness. Polishing certainly won't bother it.

I'm not sure, but I don't think the Chris had his crank rehardened or treated after stroking it .200". I really think you have nothing to worry about. You could have the crank nitrided if you want, but it won't make an important difference at it will cost you some $$.

Surface hardening on cranks has been said to be in the .0015-.003 deep range from the factory. Needless to say, wear will reduce that so polishing a used crank that is on the bottom of allowable undersize could result in polishing through the surface treatment.

Of course any .010-.030 undersized crank is well under that, and IME reman cranks are almost never re-hardened (and they've usually got a finish that looks like somebody used 80 grit sandpaper). This just allows the crank to be scored more easily by debris in the oil than a factory crank would. This results in a rougher than factory finish which in turn causes turbulent oil flow and subsequent increases in oil temp. Not to mention somewhat reduced bearing load capacity.

In you case, if it's gotta be polished it's gotta be polished and that's all there is to it. So get it done and enjoy. It's not like it'll break anytime soon just because the crank may not have quite the wear resistance it once did.